Vinylidene chloride-based latexes have an important use in that the latexes are applied to films of plastics, e.g., polypropylene, polyesters, and nylons, directly or after application of an anchor coat to thereby greatly improve the gas and water vapor barrier properties of the plastic films.
These latexes are required to form a coating film having satisfactory adhesion to base films. Although the coating film formed must be satisfactory in other properties, including printability and suitability for laminating to other films, the most strongly required property of the coating film formed is excellent gas and water vapor barrier properties.
Such a coating film is used mainly for food packaging. Depending on the kinds of foods, the packages should be immersed in hot water to sterilize the contents after packaging. In this case, the hot water treatment may impair the transparency of the coating film to make the same milky (hereinafter referred to as "boil blushing") or reduce the gas barrier properties of the coating film. There is hence a desire for a latex capable of forming a coating film which suffers from neither boil blushing nor a decrease in gas barrier properties through hot water treatment and also has excellent hot-water resistance.
For example, an examined Japanese patent publication 48-10941 discloses a technique for obtaining a latex which forms a coating film excellent in both gas barrier properties after hot water treatment and also in other properties. The technique comprises dialyzing a vinylidene chloride-based latex with a diaphragm to reduce the total content of inorganic salts to 0.5% to 0.1% to thereby produce the desired latex.
However, the example given in the above reference is insufficient in improving coating films in boil blushing.
An important requirement for food-packaging materials is to maintain the aesthetic appearance of articles. In particular, since boil blushing of a coating film significantly impairs the commercial value of the article, it is essential in the field of food packaging to use a technique for preventing the blushing. In addition, the performance required in the food-packaging market has become increasingly high in recent years, and a further improvement is necessary for markedly improving resistance to hot water.
An object of the present invention is to provide a vinylidene chloride-based latex which gives a coating film having satisfactory resistance to hot water without the disadvantages of prior art latexes, and to provide a process for producing the vinylidene chloride-based latex. More particularly, the object of the present invention is to provide a vinylidene chloride-based latex capable of providing a coating film which undergoes neither boil blushing nor a decrease in gas barrier properties through hot water treatment, and to provide a simple and efficient process for industrially producing the latex.